I need to quantify growth of an algal culture. I count the cell-number at the beginning of my exposure (N0) and at the end of the experiment (Nt). Because of my experimental setup, I can not count cells in-between. If I could, I would determine a growth rate by doing a linear regression of a log-plot of my cell-nr over time and the slope is then my growth rate. However, with two points only, a regression is senseless. It comes out like: growth-rate= ln (Nt/No)/t But since I don't really know how the cell number increases over time (rate), that approach is not exactly correct.

Now, I was wondering if you know of any other way to calculate growth based on N0 and Nt? What I came up with is that...and I am wondering what do you think is the most accurate of these :

Of course it's not much accurate to make "regression" from two points but if it follows more-less semi-log growth (can you check it at least once, without the substances you're testing or whatever), than you can make as much magic as you want, but you will still have two numbers.